1. Field of the Invention
The present invention relates to a permanent magnet type synchronous motor.
2. Description of the Related Art
Conventional permanent magnet type synchronous electric motors employ permanent magnets as the magnetic poles of a rotor, around which a stator is disposed. The stator has a plurality of teeth that face the rotor. When the coils formed on the stator are energized, a magnetic flux is induced by the current through the coils, creating repelling and attracting forces between the stator and the rotor which cause the rotor to rotate.
Since slots are formed between the teeth and the permanent magnets on the rotor are spaced from each other, the magnetic flux passing through a tooth will pass through the neighboring tooth in the next moment as the rotor rotates. When an edge of a permanent magnet passes by a slot, the magnetic flux becomes discontinuous. Thus the magnetic reactance changes so that the force of attraction between the stator and the rotor fluctuates, resulting in cogging.
To suppress cogging, Japanese patent application laid-open No. HEI-1-286758 discloses a rotor having protrusions extending radially outward between the permanent magnets. The relationship between the width A of the protrusions and the slot pitch B may be represented as follows: EQU A=(n+1/2).times.B
where n is a constant.
In a case where the opposite ends of each of the rotor protrusions have holding portions or lugs for holding the permanent magnets, the magnet holding portions are included in the measurement of the protrusion width A.
However, this conventional motor suffers from cogging, i.e. variations in motor torque caused by variations in magnetic flux due to the differences in alignment of the rotor and stator teeth at various positions of the rotor.